Chapter 21 Flashcards
What is a buffer solution?
A system that minimises pH changes when small amounts of an acid/base is added
What do buffer solutions contain?
1) weak acid
2) conjugate base
What does the weak acid, HA in a buffer solution do?
removes added alkali
What does the conjugate base, A⁻ in a buffer solution do?
removes added acid
What happens when alkalis and acids are added to a buffer?
1) two components in buffer solution react + are used up
2) solution loses buffering ability towards added acid/alkali when one component has fully reacted
Does the pH of a buffer solution change by a small/big amount?
small amount
What are the 2 ways for preparing weak acid buffer solutions?
1) preparation from a weak acid and its salt
2) preparation by partial neutralisation of the weak acid
How can a buffer solution be prepared from a weak acid and its salt?
1) mix a solution of carboxylic acid with a solution of one of its salts
2) when the carboxylic acid is added to water, the acid partially dissolves and the amount of carboxylate ions in solution is very small
What types of compounds are salts of weak acids?
ionic compounds
What happens when the salt is added to water?
it completely dissolves
salt of weak acid + water –>
carboxylate ion + salt ion
How can a buffer solution be prepared by partial neutralisation of the weak acid?
1) adding an aqueous solution of an alkali to an excess of the weak acid
2) weak acid is partially neutralised by alkali, forming conjugate base
3) some weak acid is left unreacted
When preparing a buffer solution by partial neutralisation of the weak acid, what does the resulting mixture contain?
1) salt of the weak acid
2) any unreacted weak acid
What controls the pH in an acid buffer solution?
the conjugate acid-base pair, HA (aq)/A⁻ (aq)
How does the conjugate base remove added acid?
1) [H⁺(aq)] increases
2) H⁺(aq) ions react with conjugate base, A-(aq)
3) equilibrium position shifts to left, removing most of H⁺ ions
How does the weak acid remove added alkali?
1) [OH⁻(aq)] increases
2) small concentration of H⁺(aq) ions reacts with OH⁻(aq) ions:
H⁺(aq) + OH⁻(aq) –> H₂O(l)
What is different about the buffer solutions of different weak acids?
buffer solutions operate over different pH ranges
When is a buffer most effective at removing either added acid/alkali?
when there are equal concentrations of the weak acid and conjugate bae
When [HA(aq)] = [A⁻(aq)]:
1) pH of buffer solution is the same as pKₐ value of HA
2) operating pH is typically over 2 pH units, centred at pH of pKₐ value
How can the pH of the buffer solution be fine-tuned?
by adjusting the ration of the concentrations of weak acid and conjugate base
What is the equilibrium expression for a weak acid?
HA(aq) ⇌ H⁺(aq) +A⁻(aq)
What is the Kₐ expression for a weak acid?
[H⁺(aq)] [A⁻(aq)] / [HA(aq)]
When calculating the pH of a weak acid, what approximation do you make?
[H⁺(aq)] = [A⁻(aq)}
Why is the approximation [H⁺(aq)] = [A⁻(aq)} not true for a buffer solution?
A⁻(aq) has been added as one of the components of the buffer
What pH does blood plasma need to be maintained between?
7.35-7.45
What is the pH of blood plasma controlled by?
1) a mixture of buffers
2) carbonic acid-hydrogencarbonate buffer (H₂CO₃/HCO₃⁻)
What should the pH of normal healthy blood be?
7.40
What condition develops when the pH of blood plasma falls below 7.35?
acidosis
What are the symptoms of acidosis?
1) fatigue
2) shortness of breath
3) shock
4) death
What condition develops when the pH of blood plasma rises above 7.45?
alkalosis
What are the symptoms of alkalosis?
1) muscle spasms
2) light-headedness
3) nausea
What happens to the carbonic acid-hydrogencarbonate bufferon addition of an acid, H⁺(aq)?
1) [H⁺(aq)] increases
2) H⁺(aq) ions react with conjugate base, HCO₃⁻(aq)
3) equilibrium position shifts to left, removing most H⁺(aq) ions
What happens to the carbonic acid-hydrogencarbonate bufferon addition of an alkali, OH⁻(aq)?
1) [OH⁻(aq)] increases
2) small concentration of H⁺(aq) ions reacts with OH⁻(aq) ions:
H⁺(aq) + OH⁻(aq) –> H₂O(l)
3) H₂CO₃ dissociates, shifting equilibrium position to right to restore most H⁺(aq) ions
Does the body produce more acidic/alkaline materials?
acidic
What do acid-base titrations use to monitor neutralisation reactions accrately?
indicators
What are the key features of a pH titration curve?
1) excess of acid - pH increases slowly as basic solution is added
2) equivalence point - centre of vertical section of pH titration curve
2) vertical section - pH increases rapidly on addition of small volume of base. acid and base concentrations similar
3) excess of base - pH increases slowly as basic solution is added
What is the equivalence point?
the volume of one solution that exactly reacts with the volume of the other solution
Is an acid-base indicator a strong or weak acid?
weak acid, HA, that has a distinctively different colour from its conjugate base, A⁻
What does the sensitivity of an indicator depend upon?
1) indicator itself
2) eyesight
What indicator should you use in a titration?
one that has a colour change which coincides with the vertical section of the pH titration curve
What is the end point of a titration?
when the indicator contains equal concentrations of HA and A⁻, and the colour will be in between the two extreme colours
For which titration is no indicator suitable?
weak acid-weak base
Why is no indicator suitable for a weak acid-weak base titration?
there is no vertical section and, even at its steepest, the pH requires several cm³ to pass through a typical pH indicator of 2 pH units
